Nature:早期哺乳動物在分類上的複雜性

有關來自中國侏羅紀（距今約1.60億-1.65億年前）的兩個新發現的化石的獨立報告，產生了對哺乳動物起源的相互衝突的重建結果。Haramiyids是中生代哺乳動物，具有奇特的、高度衍生的、與嚙齒類動物相似的牙齒。由於這一點，它們過去一直被認為與「多瘤齒獸類」是同類，後者是一類體型較大的、高度成功的、與嚙齒類相似的哺乳動物，直到始新世才滅絕。Haramiyids所存在的問題是，它們直到不久前還僅僅通過牙齒而為人們所知。由Jin Meng及其同事發表的一篇報告顯示了一個要完整得多的生物（形象），其特徵說明它們與「多瘤齒獸類」是同類，從而證實了早先的觀點，但也說明現存哺乳動物的起源可能遠在「三疊紀」。相比之下，由Zhe-Xi Luo及其同事描述的Haramiyids在下巴和腳踝的很多特徵上的原始程度令人吃驚，說明Haramiyids在哺乳動物這個分支上是很原始的，與「多瘤齒獸類」根本沒有關係。二者觀點的這種截然對立說明了我們對化石的了解是多麼得少，而對化石的解讀是了解哺乳動物早期演化的關鍵。（生物谷 Bioon.com）

生物谷推薦的英文摘要

Nature  doi:10.1038/nature12429

A Jurassic mammaliaform and the earliest mammalian evolutionary adaptations

Chang-Fu Zhou,  Shaoyuan Wu,  Thomas Martin  & Zhe-Xi Luo

The earliest evolution of mammals and origins of mammalian features can be traced to the mammaliaforms of the Triassic and Jurassic periods that are extinct relatives to living mammals. Here we describe a new fossil from the Middle Jurassic that has a mandibular middle ear, a gradational transition of thoracolumbar vertebrae and primitive ankle features, but highly derived molars with a high crown and multiple roots that are partially fused. The upper molars have longitudinal cusp rows that occlude alternately with those of the lower molars. This specialization for masticating plants indicates that herbivory evolved among mammaliaforms, before the rise of crown mammals. The new species shares the distinctive dental features of the eleutherodontid clade, previously represented only by isolated teeth despite its extensive geographic distribution during the Jurassic. This eleutherodontid was terrestrial and had ambulatory gaits, analogous to extant terrestrial mammals such as armadillos or rock hyrax. Its fur corroborates that mammalian integument had originated well before the common ancestor of living mammals.

Nature  doi:10.1038/nature12353

A new arboreal haramiyid shows the diversity of crown mammals in the Jurassic period

Xiaoting Zheng,  Shundong Bi,  Xiaoli Wang  & Jin Meng

A major unsolved problem in mammalian evolution is the origin of Allotheria, including Multituberculata and Haramiyida1, 2, 3, 4, 5. Multituberculates are the most diverse and best known Mesozoic era mammals and ecologically resemble rodents, but haramiyids are known mainly from isolated teeth, hampering our search for their phylogenetic relationships. Here we report a new haramiyid from the Jurassic period of China, which is, to our knowledge the largest reported so far. It has a novel dentition, a mandible resembling advanced multituberculates and postcranial features adapted for arboreal life. Our phylogenetic analysis places Haramiyida within crown Mammalia, suggesting the origin of crown Mammalia in the Late Triassic period and diversification in the Jurassic, which contrasts other estimated divergence times of crown Mammalia6, 7, 8. The new haramiyid reveals additional mammalian features of the group, helps to identify other haramiyids represented by isolated teeth, and shows again that, regardless of various phylogenetic scenarios, a complex pattern of evolution involving many convergences and/or reversals existed in Mesozoic mammals.